The Skin’s Immune System
The skin is the largest organ in area. With the Langerhans cells in the lowest epidermal layers, it is equipped with specialized immunologically competent cells. The Langerhans cells play a central role in the skin's immune system and are an integral part of the body's total defence system.
The body's own defence against microorganisms begins directly at the skin surface. Special fatty acids from the sebaceous glands (i) and the secretions of certain bacteria belonging to the physiological skin flora inhibit the growth of fungi and bacteria. Certain enzymes present in sweat (lysozymes) can destroy the cell walls of invading bacteria. If a foreign body passes this first line of defence - for example, due to skin damage - the skin's immune system responds. Many cells help fend off foreign bodies. Among these are cells - like the Langerhans cells - that are specific to the skin's immune system.
Origin and physiology of the Langerhans cells
The dendritic Langerhans cells originate in the bone marrow. They migrate to the epidermis where they form a regularly arranged network reaching a density of some 700 to 800 cells per square millimetre.
They are the furthest "outposts" of the immune system and together with macrophages (i) and granulocytes belong to the myeloid cells.
Recognizable in the electron microscope image are the very characteristic intracellular cytoplasmic organelles resembling tennis rackets, the Langerhans granules. They play an important role in receptor-specific endocytosis processes.
Functions of the Langerhans cells
The Langerhans cells specifically activate dormant T-helper cells (i) and thus initiate a primary T-cell dependent immune response. Therefore they play an important role in contact allergies, the rejection of skin grafts and other immunological processes in the skin.
After contact with the corresponding antigens (viruses, contact allergens, skin grafts) the Langerhans cell leaves the epidermis and reaches a lymph node via the lymphatic system. On its journey the Langerhans cell undergoes a maturation process leading to the presentation of the antigen on its surface. The migrating cells are replaced by a corresponding number of new Langerhans cells from the bone marrow.
In the lymph nodes the mature Langerhans cells activate the T-helper cells (i) that have the matching antigen-specific receptors on their surface. In this way they initiate the systemic immune response.
External influences on the skin's immune system
Factors influencing the activity of the Langerhans cells in the epidermis include the following:
- Cellular messengers (cytokines) such as interleukin-10
- UV radiation
- Immunosuppressive drugs (for example, corticoids)
After intensive UV exposure it was observed that the Langerhans cells retract their dendritic cell protuberances and leave the epidermis. In addition, interleukin-10 (IL-10), which is released by the skin cells when exposed to UV radiation, impairs the function of the entire immune system, even in the non-irradiated areas. This creates immunosuppressed areas in the skin that give UV-damaged skin cells a chance to repair themselves and not be eliminated by a premature immune response.